How do I zero out my metrics for a particular campaign?

Some of the stats are just counts of checkmarks in the campaign contact record, like Opened, Clicked or Bounced. To get them back to zero, you just uncheck those boxes on each record.

Others are calculated based on Those checks. Click Rate is one of those. If you get the number of clicks to 0, then the click rate should go to 0%. Open rate is the same. If the number of opens is 0, then the rate should be 0%

Total Opens is just a number field. So, you can add it to any Campaign page layout, and then edit it there directly. The field is called "Unique Email Openers", and the field name is sbm__sbx_EmailUniqueOpenersRollup__c